In wireless cellular communication system, each mobile station is associated with a base station, and one base station is associated with multiple mobile stations. The mobile station sends and receives all data through the associated base station using uplink and downlink channels, respectively. The base station, in turn, is connected to other wired or wireless entities that route and forward packets between mobile stations and other network end-points. The physical and information structure of uplink and downlink channels and processes for managing them, are defined in a mobile communications protocol.
Multiple mobile stations may potentially require their packets to be sent at the same time. Thus, the protocol design must allow for multiplexing these packets. In 4G candidate protocols like LTE-A and 802.16m, bandwidth request multiplexing is achieved by assigning each mobile station a dedicated uplink request channel. A dedicated request channel effectively allows two mobile stations to request bandwidth at the same time. In this method, however, even mobile station that do not have any data to send also occupy request channels; leading to low average channel utilization, and hence wastage of communication resources. The bandwidth request multiplexing may also be achieved by resolving contention for bandwidth request on a common random access channel. A common random access channel is an effective utilization of resources; however, two mobile stations that request bandwidth together cannot get their bandwidth requests decoded by the base station due to collision.
Some techniques from 802.11 contention resolution, though, are universally applicable—like binary exponential back off (BEB). In 802.11, contentions between two mobile stations are resolved by a “back-off” and retry mechanism. Mobile stations detect contention and each contending mobile station chooses a time interval to wait before resending the data frame. The time interval that each mobile station waits before retrying is called a “back-off timer”; and its value is chosen uniformly at random from a “contention window”. A mobile station may retry multiple times in contention for sending the same data frame. The contention window increases for each retry ‘k’ by the recurrence CWk=2*CWk−1(k>1), CW1=CWmin.
The issue of differentiated resource allocation in wireless networks based on the delay sensitivities of different types of traffic has been addressed in the past. The Medium Access Control (MAC) layer of the IEEE 802.11 standard uses the Distribution Coordination Function (DCF), which employs the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) algorithm and optional 802.11 Request-To-Send/Clear-To-Send (RTS/CTS) control frames, to share the medium between multiple stations. This technique does not differentiate between different types of traffic and as a result, a delay sensitive type, Voice over Wireless LAN (VoWLAN) for example, might experience an intolerably long delay as compared to another type that is not so delay-sensitive, e-mail for example, that might get quicker service. The IEEE 802.11e-2005 amendment to the 802.11 standard addresses this limitation by proposing modifications to the MAC layer. It describes an Enhanced Distributed Channel Access (EDCA) scheme which defines Traffic Categories (TC). For example, e-mail could be assigned to a low priority class and VoWLAN could be assigned to a high priority class. With EDCA, high priority traffic has a higher chance of being sent than low priority traffic: a station with high priority traffic waits a little less before it sends its packet, on average, than a station with low priority traffic.
It has already been proposed that a typical bandwidth request channel is shared by all mobile station users. Simultaneous bandwidth requests by multiple users collide, and cannot be decoded by the base station. A mobile station that does not receive a response from the base station until a certain timeout period must assume that its request collided, and hence must retry sending the request. Each retry attempt is made after waiting for a random backoff timer to reduce the probability of repeated collisions between users. This may occur a few times until the mobile station either gets a response or gives up trying. In most communication systems, the back-off timer is a random number typically drawn uniformly from the interval (0, max), where max is exponential in the number of failed attempts. However, the scheme as described above suffers from QoS insensitivity. Here, users with different QoS requirements have equal chance of winning contention for their bandwidth requests. The above scheme also suffers from delay inequity. Users that have attempted more times have a larger expected back-off timer than users that have attempted fewer times. Thus, the expected delay in getting service increases with the number of failed request attempts.